1. Field of the Invention
The present invention relates to a vibrational wave driving apparatus adopting a vibrator that excites driving vibration in an elastic member using an electromechanical energy transducing element as a driving source.
2. Description of the Related Art
Because a vibrational wave driving apparatus vibrates an elastic member using stress generated by an electromechanical energy transducing element and directly performs friction drive on a driven body that is in friction contact with the elastic member, the vibrational wave driving apparatus does not require magnetism as a driving principle thereof. For this reason, the vibrational wave driving apparatus may be composed of a nonmagnetic material and does not significantly affect a magnetic field. Therefore, the vibrational wave driving apparatus is used in an electron beam drawing apparatus that uses a weak magnetic field or a magnetic resonance imaging (MRI) apparatus that operates in a strong magnetic field environment.
In Japanese Patent Laid-Open No. 2002-142473, a vibrational wave driving apparatus is disclosed in which a flexible circuit board having a multilayer structure whose terminal is in contact with electrodes of a piezoelectric element. In Japanese Patent Laid-Open No. 2004-320846, a vibratory driving apparatus that drives a driven body in a linear direction using a combination between two types of bending vibration is disclosed.
Even when a vibrational wave driving apparatus is composed of a nonmagnetic material, electric energy needs to be supplied to a piezoelectric element for driving the vibrational wave driving apparatus. Therefore, current flowing through a wire extending from a power supply apparatus to the piezoelectric element and a wire electrically connecting electrodes on surfaces of a piezoelectric body can generate magnetic noise. When the vibrational wave driving apparatus is used in an electron beam drawing apparatus or an MRI apparatus, which is susceptible to a weak magnetic field, there is a problem in that the magnetic noise adversely affects the operation of the apparatus.
With respect to the wire extending from the power supply apparatus to the piezoelectric element, the magnetic noise may be reduced by using a twisted pair cable or a coaxial cable. On the other hand, with respect to the arrangement of the electrodes of the piezoelectric element, the degree of freedom of the arrangement is low because the electrodes are arranged in accordance with restrictions imposed by the driving principle. The arrangement of the electrodes of the piezoelectric element is determined such that desired driving vibration is excited in an elastic member, and then a flexible printed circuit board is connected to the electrodes. Because driving regions formed in the piezoelectric element corresponding to the plurality of element electrodes are capacitive loads, that is, capacitors, each driving region generates a charge proportional to driving voltage. Furthermore, each driving region generates a charge in accordance with strain caused by mechanical vibration of the piezoelectric element due to a piezoelectric phenomenon. When a driving circuit is formed by connecting the element electrodes using the flexible printed circuit board and driving voltage is applied, current based on the charge caused by the capacitors and the charge caused by the piezoelectric effect flows through wires in the flexible printed circuit board.
When changes caused by minute variations in a magnetic field are detected such as in the case of an MRI apparatus, a magnetic field generated by the above-described current causes variations in a magnetic field of a portion to be measured, a detection signal and a resultant diagnostic image may be affected.